I love August, and I actually love Mackenzie on a boy, I can see where you could think it was feminine but I love your combo. Some other names you might consider....
(I'll do my best with the pronunciation issue)

Although I see August as a more masculine name, I've known two in my life and both were female. Paired with Mackenzie, I would have no idea if the child was a boy or a girl...and honestly I would lean towards a girl (mostly due to Mackenzie, though!)

Since you're already using a unisex first name, I would use a definitively masculine middle - another family name, perhaps?